Binaural stimulation is common in the case of hearing aids, but it has only recently become common in the case of neural auditory prostheses, such as cochlear implants (CI). In the case of cochlear implants, binaural stimulation requires two implants, one in each ear. In the case of cochlear implants or other neural auditory prostheses, the incoming left and right side acoustic signals are similar to those in hearing aids and may simply be the output signals of microphones located in the vicinity of the left and right ear, respectively. However, unlike hearing aids, in neural auditory prostheses, the output is in the form of electrical stimulation signals for directly stimulating auditory nerve tissue. In the following, the discussion with regard to neural auditory prosthesis is focused on cochlear implants for illustrative purposes, but it is to be understood that everything mentioned with specific reference to cochlear implants may equally apply for brain stem implants or modiolar implants, without further mention.
Bilateral cochlear implants provide the benefits of two sided hearing which allows a listener to localize sources of sound in the horizontal plane. That requires information from both ears such as interaural level differences and interaural time differences (ITDs). This is discussed further, for example, in Macpherson, E. A, and Middlebrooks, J. C., Listener Weighting Of Cues For Lateral Angle: The Duplex Theory Of Sound Localization Revisited, J. Acoust. Soc. Am. 111, 2219-3622, 2002, which is incorporated herein by reference. An ITD is a relative time shift between signals arriving at the left and right ear which is caused by different times for the signal to reach each ear when the source of sound is not within the median plane. Two-sided hearing also is known to male speech easier to understand in noise, and again the perception of ITD plays a pivotal role therein. This is explained more fully, for example, in Bronkhorst, A. W., and Plomp, R., The Effect Of Head-Induced Interaural Time And Level Differences On Speech Intelligibility In Noise, J. Acoust. Soc. Am. 83, 1508-1516, 1988, which is incorporated herein by reference.
In the perception of ITDs, one can distinguish two sources of ITD information, namely ITD information from the envelope of a signal and ITD information from the fine structure of a signal. With reference to FIG. 7, an oscillatory signal can be characterized by its fine structure, i.e. information on the level of the fast varying oscillatory signal 10 and a slowly varying envelope 12. Fine structure information of a signal can, for example, be reflected in the timings of the peaks or zero crossing of the rapidly oscillating signal.
It has been found that of the envelope ITD information and the fine structure ITD information, the latter one plays a more important role for sound localization and for understanding of speech in noise. This has been shown, for example, in Wightman and Kistler, Factors Affecting The Relative Salience Of Sound Localization Cues in Binaural and Spatial Hearing in Real and Virtual Environments, edited by Gilkey, R. H., and Anderson, T. R., (Lawrence Erlbaum Associates, Mahwah, N.J., 1997); Smith et al., Chimaeric Sounds Reveal Dichotomies In Auditory Perception, in Nature 416, 87-90, 2002; Nie et al., Encoding Frequency Modulation To Improve Cochlear Implant Performance In Noise, IEEE Trans. Biomed. Eng. 52, 64-73, 2005; and Zeng et al., Speech Recognition With Amplitude And Frequency Modulations, Proc. Natl. Acad. Sci. 102, 2293-2298, 2005, all of which are incorporated herein by reference, 2005, all of which are incorporated herein by reference.
In conventional CIs, fine structure information is not used. Instead, the incoming sound is separated into a number of frequency bands, for each band the slowly-varying envelope is extracted, and this envelope information is used to modulate the amplitude of a high-frequency pulsatile carrier signal. In such conventional CIs, the frequency and phase of the pulsatile carrier signal is simply dictated by the speech processor and not directly related to the fine structure of the incoming signal. Accordingly, with such known CIs, only the envelope ITD information is available, and consequently, ITD perception is very limited.
Recently, CIs have been proposed in which the stimulation signals are comprised of stimulation pulses with a timing that is based on temporal events within the fine structure of the left and right side acoustic signals. For instance, such temporal events could be the peaks or zero crossings within the fine structure of the signal. Stimulation schemes for coding fine structure information have been suggested for example by U.S. Patent Publication 20040478675, U.S. Pat. No. 6,594,525; U.S. Patent Publication 2004136556; which are incorporated herein by reference, and in van Hoesel and Tyler, Speech Perception, Localization, And Lateralization With Bilateral Cochlear Implants, J. Acoust. Soc. Am. 113, 1617-1630, 2003; and Litval (et al., Auditory Nerve Fiber Responses To Electric Stimulation: Modulated And Unmodulated Pulse Trains, J. Acoust. Soc. Am. 110(1), 368-79, 2001, also incorporated herein by reference. With these improved stimulation strategies, one would have expected that the ITD perception should be increased as compared to stimulation strategies comprising envelope ITD information only. However, in comparative studies no improvement in sound localization or in the understanding of speech in noise environments could be found; See van Hoesel supra.
Hearing aid listeners are also known to have difficulties with localizing sources of sound and understanding of speech in noisy environments. See for example, Colbum, S. et al. Binaural Directional Hearing—Impairments And Aids in W. Yost & G. Gourevitch (Eds.), Directional Hearing pp. 261-278, New York: Springer-Verlag, 1987; Durlach N. I. et al., Binaural Interaction Of Impaired Listeners. A Review Of Past Research in Audiology, 20(3):181-211, 1981; Gabriel K. J. et al. Frequency Dependence Of Binaural Performance In Listeners With Impaired Binaural Hearing, J Acoust Soc Am., Jan: 91(1):336-47, 1992; Hawkins D B, Wightman F L. (1980). Interaural time discrimination ability of listeners with sensorineural hearing loss. Audiology. 19, 495-507; Kinkel, M. et al., Binaurales Hören bei Normalhörenden und Schwerhörigen I. Meβmethoden und Meβergebnisse, Audiologische Akustik 6/91, 192-201, 1991; Koelnike, J. et al., Effects Of Reference Interaural Time And Intensity Differences On Binaural Performance In Listeners With Normal And Impaired Hearing, Ear and Hearing, 16, 331-353, 1995; and Smoski, W. J. and Trahiotis, C., Discrimination Of Interaural Temporal Disparities By Normal-Hearing Listeners And Listeners With High-Frequency Sensorineural Hearing Loss, J Acoust Soc Am. 79, 1541-7, 1986, all of which are incorporated herein by reference.